Inside bulb coating for ultraviolet lamp

ABSTRACT

A transparent layer of aluminum oxide is provided on the inside surface of the glass bulb of a gas discharge ultraviolet germicidal lamp. This results in improved starting, particularly under low line voltage conditions, and reduced bulb discoloration during the operating life of the lamp. To make the transparent layer, a coating is applied to the bulb, preferably in the form of aluminum S butoxide, combined with other chemicals to retard premature hydrolysis and/or gelling. After air drying, the coated bulb is lehred to produce the final coating of aluminum oxide.

United States Patent Hammer [4 1 Oct. 29, 1974 INSIDE BULB COATING FORULTRAVIOLET LAMP 3,fil7,357 ll/l97l Nagg 313/22] Primary ExaminerHermanKarl Saalbach Assistant ExaminerDarwin R. Hostetter Attorney, Agent, orFirm-Norman C. Fulmer;

Laurence R. Kempton; Frank L. Neuhauser [57] ABSTRACT A transparentlayer of aluminum oxide is provided on the inside surface of the glassbulb of a gas discharge ultraviolet germicidal lamp. This results inimproved starting, particularly under low line voltage conditions, andreduced bulb discoloration during the operating life of the lamp. Tomake the transparent layer, a coating is applied to the bulb, preferablyin the form of aluminum S butoxide, combined with other chemicals toretard premature hydrolysis and/or gelling. After air drying, the coatedbulb is lehred to produce the final coating of aluminum oxide.

3 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION The invention isin the field of gas discharge ultraviolet lamps, such as germicidallamps, and is particularly directed to a beneficial coating on theinside surface of the bulb.

Gas discharge ultraviolet lamps for germicidal purposes are generallysimilar to conventional fluorescent lamps in that they comprise a pairof spaced apart electrodes near the ends of an elongated glass bulb, andthe bulb contains a small amount of mercury to provide a mercury-vapordischarge path between the electrodes which generates ultravioletradiation. The bulb may also contain argon or other gas to aid instarting the discharge. Unlike fluorescent lamps, however, germicidallamps usually do not contain a phosphor for converting the ultravioletradiation into visible light. Also, the bulbs of germicidal lamps havesubstantially higher concentrations of BaO in their composition whichhelps to increase the transmission of 2,537 A germical radiation.whereas fluorescent lamp bulbs are usually made from soft lime glasscontaining sodium alkali and absorb the ultraviolet but transmit thevisible light thereby preventing harmful effects which the ultravioletradiation would cause to people.

US. Pat. No. 2,445,692 to Lawrence Porter and Herman Froelich,Ultraviolet Ray Generator," describes an ultraviolet lamp whichgenerates short ultraviolet radiation directly from the mercurydischarge for germicidal purposes, and which contains a phosphor on theinner surface of the bulb for converting some ofthe short ultravioletinto middle range" ultraviolet radiation for erythemal health-raypurposes. Such a lamp is particularly useful in poultry barns. US. Pat.No. 2,295,626 to Norman Beese describes an ultraviolet lamp having aglass bulb containing some alkali, and the inner surface of the bulb iscoated with pure silica so that, as seen by the mercury-vapor dischargeinside the lamp, the bulb is the equivalent of an alkali-free glass andthere is no problem of mercury amalgamating with alkali from the bulbmaterial. Such a bulb is intended to be included in the termalkali-free" glass bulb as used hereinv SUMMARY OF THE INVENTION Objectsof the invention are to provide an improved gas discharge ultravioletlamp, and to provide such a lamp having improved and reduced startingvoltage characteristics and also having a reduced tendency for bulbdiscoloration to occur during the operating life of the lamp.

The invention comprises, briefly and in a preferred embodiment, atransparent layer of aluminum oxide (Ala) on the inside surface of anultraviolettransmitting glass bulb of a gas discharge germicidal lamp.In a preferred method, the inside bulb surface is coated with a solutionof aluminum S butoxide [AL iO(,H,. t, combined with acetic acid,butanol, a thinner, and a binder, and the coated bulb is lchred to formthe linal {AI-Z01 layer. The term "glass bulb" as used herein isintended to include bulbs of quartz and other suitable vitreousmaterials or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE ofthe drawing is abroken-away perspective view of a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis shown a fluorescent lamp 1 comprising an elongated ultraviolettransmitting germicidal glass bulb 2 of circular cross section andessentially free of sodium alkali. The lamp has the usual electrodestructure 3 at each end supported on in-lead wires 4, 5 which extendthrough a glass press 6 in a mount stem 7 to the contacts ofa base 8affixed to the end of the lamp. The sealed bulb is filled with an inertgas such as argon or a mixture of argon and other gases at a lowpressure, for example about 3 torr, and a small quantity of mercury, atleast enough to provide a low vapor pressure of about 6 microns (6 torr)during lamp operation. The inner surface of the glass bulb is providedwith a thin transparent layer 9 of aluminum oxide (Al o This film isessentially continuous and non-particulate and has a thickness ofbetween 0.1 and I0 microns, the thickness preferably lying in the rangebetween 0.2 and 0.5 microns. If the film thickness exceeds one micron,some of the beneficial 2,537 A germicidal radiation is absorbed by theAIgOg layer, resulting in a less efficient germicidal lamp.

The layer may be formed by applying to the inner surface ofthe bulb 2,prior to sealing in the end mounts 6 and 7, a coating of the followingcomposition:

ct. 3 cc 5 cc l4ll cc 20 cc 50% hutyl acetate] The coated bulb is thenlehred in the usual way at a temperature of about 550 to 600 C forapproximately 3 to 5 minutes to decompose the composition and convert itinto A1 0 It is found that the invention achieves desired improvementsof lowering the starting voltage of the lamp and also reducing thetendency for bulb discoloration to occur during operation of the lamp.For example, when the invention is applied to type G8T5 germicidallamps, their test table starting voltages average about It) to 20 voltslower than the standard uncoated germicidal lamps. While the exactnature of these improvements is not fully understood, it is believedthat the AI O internal layer on the bulb surface acts as a getter forabsorbing any water vapor, CO oxygen, or other stray gases that may bewithin the lamp.

Aluminum oxide layers have been used or proposed for use on the innersurface of fluorescent lamp bulbs made of sodium alkali-bearingcompositions, and the purpose of the aluminum oxide layer in such lampsis to act as a barrier between the mercury contained in the lamp and thesodium alkali component of the glass bulb, to prevent the formation ofundesirable amalgams ofthe mercury with the sodium alkali in the bulbmaterial. Examples of such constructions are described in US. Pat. Nos.3,067,356; 3,l4l,990; and 3,205,394 to John Ray; in US. Pat. No.3,54l,377 to Rudolph Nagy; and in US. Pat. No. 3,599,029 to WilliamMartyny.

However, such combinations of aluminum oxide layers on sodiumalkali-type glass bulbs. for the purpose of acting as a barrier toprevent amalgamation of mercury and sodium alkali, is a differentcombination, and performs a different function, than the inventiondisclosed herein of providing a layer of aluminum oxide on the innersurface of an essentially sodium'free glass bulb for the purpose ofachieving a lower starting voltage for the ultraviolet lamp and alsoreducing discoloration of the bulb during lamp operation. Also. thealuminum oxide layer in the above-mentioned fluorescent lamp bulbs is ofa particulate type having discrete particles of aluminum oxide, and isof necessity of greater thickness, to perform its function as a sodiumalkali barrier. than the thin smooth, continuous and non-particulatealuminum oxide layer of the present invention.

While preferred embodiments of the invention have been shown anddescribed, various other embodiments and modifications thereof willbecome apparent to persons skilled in the art. and will fall within thescope of the present invention as defined in the following claims.

ness is between 0.2 and 0.5 microns.

1. A GAS DISCHARGE ULTRAVIOLET LAMP COMPRISING A BULT OF SUBSTANTIALLYSODIUM-FREE ULTRAVIOLET-TRANSMITTING GLASS AND CONTAINING MERCURY VAPORAT LOW PRESSURE WHEREIN THE IMPROVOEMENT COMPRISES A LAYER OFNON-PARTICLATE ALUMINUM OXIDE (AL2O3) ON THE INNER SURFACE OF THE BULB.2. A lamp as claimed in claim 1 in which said layer of aluminum oxide isa non-particulate continuous film having a thickness between 0.1 and 1.0microns.
 3. A lamp as claimed in claim 2 in which said thickness isbetween 0.2 and 0.5 microns.